1. Field of the Invention
This invention relates to a method for and an apparatus for use in testing a circuit breaker and, more particularly to a method for and an apparatus for use in testing a circuit breaker which can provide an indication of the condition of the entire trip system in a circuit breaker, as well as provide an indication of the condition of an already installed circuit breaker.
2. Description of the Prior Art
Often, it is desirable and necessary to test circuit breakers that have been already installed. The purpose of testing the circuit breaker is to verify that the circuit breaker will actually trip at the proper current values and with the proper time delay. Having a circuit breaker trip at the appropriate current values and with the proper time delay can prevent or minimize damage to the device being protected by the breaker upon occurrence of an overcurrent condition.
A generally accepted method of testing circuit breakers is known as primary injection testing. In primary injection testing, a current from a low voltage source is passed directly through each breaker pole in a circuit breaker. Primary injection testing generally requires a power supply which can produce several times rated breaker current, which for various circuit breakers could amount to thousands of amperes. To conduct the test, the conductors which normally carry the load current must be removed from the circuit breaker and heavy test leads are connected in their place to energize the circuit breaker. The primary injection testing process normally requires a significant expenditure of effort and time. Also, a relatively high current from an alternating current source is generally required. A potential safety hazard is presented in this method of testing, since a risk may exist to personnel and equipment when the poles of a circuit breaker are connected to or disconnected from a potentially energized electrical conductor.
On circuit breakers having solid state trip units, a simpler method of testing over the primary injection testing method is known. In this method, a low power direct current source injects one of a number of preset values of direct current into the trip circuit of a circuit breaker to simulate an overcurrent condition. Depending on the preset value, the circuit breaker either trips instantaneously or after an appropriate time delay. Testing of the solid state trip circuit of a circuit breaker in this manner is advantageous in that it is not necessary to connect or disconnect energized conductors to the circuit breaker. Generally, testing of the trip circuit can be done with equipment of a relatively small volume and weight. An example, of a circuit breaker fault simulator used in testing the solid state trip circuitry of a circuit breaker is disclosed in U.S. Pat. No. 3,852,660 to Maier et al. and U.S. Pat. No. 3,924,160 to Maier et al. In these patents a relatively small, lightweight fault simulator is provided which has a selector switch thereon. The simulator is connected to terminals on the outer casing on the circuit breaker which connects the fault simulator to the trip circuit in series with the fault sensors normally associated with the trip circuit of the circuit breaker. A relatively high level simulated fault signal or a relatively low level simulated fault signal may be provided to the trip circuit by adjusting the selector switch. A disadvantage of testing only the trip circuitry of a circuit breaker in the above described manner is that the current transformers in the circuit breaker are not tested nor are the input bridge rectifiers or similar rectification devices in the solid state circuitry.
U.S. Pat. No. 4,105,965 to Russell discloses a static trip circuit breaker test set. In Russell, there is provided an improved field test set for static trip circuit breakers which is adapted to inject low current, fault simulating signals directly into the breaker static trip unit. Russell further discloses a current transformer continuity check circuit which is included in the test set. Additionally, the Russell patent also discloses a known type of field test set for testing a static trip circuit breaker whereby low current, fault simulating signals are injected into the secondary circuits of the breaker phase current monitoring transformers which are connected as inputs to the static trip unit.
U.S. Pat. No. 3,424,958 to Groat discloses an apparatus and method for simulating electrical fault conditions in a protective relay system. In Groat's method, a voltage and a current are fed to a relay to be tested. The phase angle between the voltage and the current is controlled and the magnitude of the voltage and current is caused to change from a first and a second value and the sequence of change is controlled.
U.S. Pat. No. 3,930,187 to Misencik discloses a ground fault interrupter of the differential transformer type having a means for protection against a grounded neutral condition. A test circuit branch is connected between a first point on the load side of the differential transformer to the hot line conductor and on the supply side of the differential transformer to the neutral conductor. The test circuit branch provides the capability to test the performance of the differential transformer.
What is needed, therefore, is a method for and an apparatus for use in testing a circuit breaker which requires a relatively low power alternating current source and offers a way of testing the entire circuit breaker to determine if a defective condition exists therein. What is further needed is an apparatus for use in testing the entire circuit breaker which is of a relatively small volume and weight and draws only a small current from a low voltage alternating current source.